THE EARLY TRIASSIC STEM−FROG CZATKOBATRACHUS from POLAND -.: Palaeontologia Polonica
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THE EARLY TRIASSIC STEM−FROG CZATKOBATRACHUS FROM POLAND SUSAN E. EVANS and MAGDALENA BORSUK−BIAŁYNICKA Evans, S.E. and Borsuk−Białynicka, M. 2009. The Early Triassic stem−frog Czatkobatrachus from Poland. Palaeontologica Polonica 65, 79–105. Czatkobatrachus polonicus Evans et Borsuk−Białynicka, 1998 is a stem−frog from the Early Triassic karst locality of Czatkowice 1 (southern Poland). It was described and named on the basis of a small collection of vertebrae, ilia, and forelimb bones, with subsequent description of the scapulocoracoid. Further skeletal elements have now been recovered. Here we present a complete overview of the available material of Czatkobatrachus, give an extended diagno− sis of the genus, and provide an assessment of its relevance to our understanding of the early stages of salientian evolution. Czatkobatrachus has an anuran type ilium, a urodelan−like scapulocoracoid, unfused epipodials, a moderately shortened presacral column, and a very short tail of separate caudal vertebrae. The strongly ossified ends of the proximal limb bones suggest terrestrial life. The skull is unknown. Key words: Stem−frogs, Anura, Salientia, Lissamphibia, Triassic. Susan E. Evans [[email protected]], Research Department of Cell and Developmental Bio− logy, UCL, University College London, Gower Street, London, WC1E 6BT, UK. Magdalena Borsuk−Białynicka [[email protected]], Instytut Paleobiologii PAN, Twarda 51/55, 00−818 Warszawa, Poland. Received 17 November 2006, accepted 15 September 2008 80 SUSAN E. EVANS and MAGDALENA BORSUK−BIAŁYNICKA INTRODUCTION Czatkobatrachus polonicus Evans et Borsuk−Białynicka, 1998 is a stem−frog described from the Early Triassic microvertebrate locality of Czatkowice 1 in southern Poland (Paszkowski and Wieczorek 1982). It is the earliest lissamphibian known from Northern Pangaea, and only the second recovered from Triassic de− posits, the roughly contemporaneous salientian Triadobatrachus massinoti (Piveteau 1936) from Madagas− car being the first. To date, only a few postcranial bones of Czatkobatrachus (ilia, distal humeri, ulnae, verte− brae, scapulocoracoid) have been described (Evans and Borsuk−Białynicka 1998; Borsuk−Białynicka et al. 1999; Borsuk−Białynicka and Evans 2002). Incomplete as they are, the remains of Czatkobatrachus are gen− erally similar to those of Triadobatrachus but are significantly smaller and appear more derived (or at least better ossified) in rib, vertebral structure and limb morphology. The objective of the present paper is to describe newly discovered material of Czatkobatrachus polonicus, as well as give a more detailed description of skeletal elements only briefly discussed in the original paper, most notably parts of the vertebral column and limbs. Institutional abbreviations. — MNHN, Museum National d'Histoire Naturelle, Paris, France; UCMP, University of California, Museum of Paleontology, Berkeley, USA; ZPAL, Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland. Acknowledgments. — The authors are indebted to the team of the Institute of Geology, Jagiellonian Uni− versity, Kraków, and particularly Józef Wieczorek and Mariusz Paszkowski who discovered, and generously transferred the bone breccia from Czatkowice 1 to the Museum of the Earth and the Institute of Paleobiology, Polish Academy of Sciences, Warsaw. Our thanks are due also to the late Halszka Osmolska (Institute of Paleobiology) and Teresa Maryańska (Museum of the Earth) for access to materials in their care; Jean−Claude Rage (Museum d’Histoire Naturelle, Paris), for access to high−resolution casts of the holotype of Triado− batrachus massinoti; A.G. Jacobson (University of Texas, Austin) for information on head development in am− phibians; Borja Sanchiz (Natural History Museum, Madrid) for information on vertebral development; and Carl Gans (University of Texas, Austin) and Farish Jenkins Jr (Harvard University), for discussions on the func− tional morphology of early frogs. We are grateful to the referees: F. Jenkins Jr and Zbyněk Roček (Charles Uni− versity Prague). Critical comments of the latter helped us to improve the final version of the manuscript. Ewa Hara (Institute of Paleobiology) carried out the acid preparation of the Czatkowice 1 breccia; Cyprian Kulicki (Institute of Paleobiology) performed the scanning electron microscope photography. GEOLOGICAL SETTING The Triassic deposits filling the karst fissures developed in the Early Carboniferous limestones at Czat− kowice 1 (southern Poland) were discovered by a team from the Jagiellonian University, Kraków, and were first described by Paszkowski and Wieczorek (1982). The material described herein comes from the largest of the karst fissures called Czatkowice 1. This material has been dated as probably Early Olenekian in age (Borsuk−Białynicka et al. 2003). A more detailed account of the geology may be found elsewhere in this vol− ume (Cook and Trueman 2009; Paszkowski 2009). MATERIAL AND METHODS The bone material from the karst deposits of Czatkowice 1 consists of the completely disarticulated skele− tons of several small vertebrates, most of them reptiles (Borsuk−Białynicka et al.1999). Salientian postcranial bones are easily discriminated from reptilian ones by their anatomical structure, further supported by their size range (all but one of the Czatkowice 1 reptiles are larger as adults); their frequency (Czatkobatrachus re− mains are comparatively scarce); and the type of bone tissue in terms of surface appearance. The skeletal ele− EARLY TRIASSIC STEM−FROG CZATKOBATRACHUS FROM POLAND 81 ments were compared with those of both extant and fossil frogs, particularly the earliest known Jurassic frogs: Notobatrachus degiustoi Reig, 1955 and Vieraella herbsti Reig, 1961 (Báez and Basso 1996), and Prosalirus bitis Shubin et Jenkins, 1995 (Jenkins and Shubin 1998). Most informative was a comparison with the Early Triassic Malagasy basal salientian Triadobatrachus massinoti (Piveteau, 1936) (Rage and Roček 1989; Roček and Rage 2000). The puzzling absence of skull bones cannot be explained as the effect of chemical preparation in acetic acid, because equally small and fine reptilian elements are perfectly preserved. It may be partly an artifact of the relative scarcity of Czatkobatrachus remains overall. Although all crown−group frogs have lost the lower dentition, most basal taxa, as well as many neobatrachians, retain teeth in the maxilla, and these toothed ele− ments are both common and distinctive in microvertebrate assemblages, even when fragmented. Nonethe− less, despite a careful scrutiny of all toothed elements from Czatkowice 1, none are attributable to Czatko− batrachus. Rage and Rocek (1989) found no trace of teeth in Triadobatrachus, but only the most posterior part of the maxilla is preserved, and no teeth are expected in this part (Roček personal communication 2007). The scanning electron microscope was used extensively for illustrations and studies of surface texture. Terminology used follows Sanchiz (1998). PHYLOGENETIC BACKGROUND No−one has ever seriously doubted the monophyly of Anura, and although the status of Triadobatrachus as a stem−frog has occasionally been challenged (e.g., Hecht 1960), most authors accept also the monophyly of Salientia, including Triadobatrachus, and now Czatkobatrachus, along with crown−group Anura (e.g., Duellman and Trueb 1986; Milner 1988; Rage and Roček 1989; Sanchiz 1999; Roček 2000; Carroll 2007; Anderson et al. 2008). There is also a general consensus that Salientia were derived from temnospondyl, or more precisely dissorophoid, ancestors (Bolt 1969, 1977, 1991; Bolt and Lombard 1985; Daly 1994; Milner 1988, 1990, 1993; Carroll 1999; but see Laurin and Reisz 1997; Laurin et al. 2000; Yates and Warren 2000; Anderson et al. 2008). The monophyly of the Lissamphibia as a whole (Salientia, Caudata and Gymnophiona) was proposed by Parsons and Williams (1962, 1963), Szarski (1962), and Bolt (1969), and has been supported by many au− thors (e.g., Milner 1988; Rage and Janvier 1982; Gauthier et al. 1989; Trueb and Cloutier 1991; Cannatella and Hillis 1993; Ford and Cannatella 1993; McGowan and Evans 1995; Gardner 2000). Others have rejected or questioned lissamphibian monophyly (e.g., Shishkin 1973; Bolt and Lombard 1985; Carroll and Holmes 1980; Carroll 1999; Carroll et al. 1999), and the debate is ongoing. The recovery and description of early rep− resentatives of major lissamphibian lineages are clearly critical to the discussion of their ancestry. SYSTEMATIC PALEONTOLOGY Class Amphibia Linné, 1758 Order Salientia Laurenti, 1768 Family uncertain Genus Czatkobatrachus Evans et Borsuk−Białynicka, 1998 Czatkobatrachus polonicus Evans et Borsuk−Białynicka, 1998 Holotype: ZPAL Ab IV/7, a right ilium. Type horizon and locality: Olenekian karst deposits at Czatkowice Quarry (locality 1), Kraków region, Poland. Material. — 76 catalogued specimens, including 15 presacral vertebrae, 2 sacral vertebrae, 9 scapulo− coracoids, 11 humeri, 2 ulnae, 19 ilia, 5 femora. Emended generic and specific diagnosis. — Small stem−frog (less than 50 mm snout−vent length) that resembles Triadobatrachus and all other salientians in having an anteriorly extended iliac shaft. It resembles 82 SUSAN E. EVANS and MAGDALENA BORSUK−BIAŁYNICKA Triadobatrachus and differs from crown−group anurans in retaining relatively long neural arches, unfused epipodials, a series of unfused caudal vertebrae instead of an urostyle (Lynch 1973; Trueb 1973; Sanchiz 1998), a scapulocoracoid that is a single ossification, and